Field of the Invention
The present invention relates to an image forming apparatus, for example, a digital copying machine, and more particularly, to an image forming apparatus configured to perform magnification correction of an optical system.
Description of the Related Art
In an electrophotographic image forming apparatus, for example, a digital copying machine, an image is formed by forming an electrostatic latent image on a photosensitive member through control of laser in accordance with an image signal, and by performing developing, transfer, and fixing steps. A laser beam radiated to the photosensitive member is deflected with a rotation of a rotary polygon mirror, and the photosensitive member is scanned in a longitudinal direction (hereinafter referred to as “main scanning direction”) with the laser beam. Moreover, with the rotation of the photosensitive member, scanning is performed in a direction (hereinafter referred to as “sub-scanning direction”) orthogonal to the main scanning direction, and a two-dimensional latent image is formed on the photosensitive member. Moreover, in the deflection with the rotation of the rotary polygon mirror, the laser beam is radiated to the photosensitive member through an fθ lens to perform optical correction with the fθ lens. In other words, scanning characteristics of the laser beam, such as a scanning speed, an optical path length, and an angle of incidence in the longitudinal direction are uniformized by the fθ lens.
When a simple fθ lens is used, a slight residual of the scanning characteristics that remains even after the optical correction by the fθ lens is corrected by magnification correction processing in the main scanning direction through image processing. For example, there is a method involving treating each pixel in units (hereinafter referred to as “divided pixels”) obtained by dividing one pixel in the main scanning direction, and converting a gradation of each pixel through pulse width modulation (PWM) (Japanese Patent Application Laid-Open No. 2013-022913). This method is a method of for suppressing a degradation in image quality by subjecting image data that has been converted through PWM to interpolation processing with a high frequency in units of a divided pixel. Positions (hereinafter referred to as “insertion-extraction positions”) at which divided pixels are inserted or extracted through the interpolation processing occur substantially at fixed intervals in the main scanning direction for a fixed magnification. In order to prevent moire caused by interference between a period of the insertion-extraction positions of the divided pixels and a PWM period, the insertion-extraction positions are controlled to reduce occurrence of a local difference in density.
Meanwhile, as the optical structure without the fθ lens in pursuit of a low cost, there has been proposed a method of performing magnification correction entirely with electric correction (Japanese Patent Application Laid-Open No. 2004-338280). In such method, the magnification correction is performed by dividing the main scanning direction into predetermined areas, and modulating a clock frequency in accordance with a magnification in each area. A low-cost optical system may be realized with a configuration in which a PWM signal is controlled in magnification with the optical structure without the fθ lens.
However, in the related-art method, there are problems of an increased hardware scale for correction processing and a reduction in image quality. As illustrated in FIG. 8A, in the structure without the fθ lens, a scanning v(θ) with the laser beam is not constant, and depends on an image height, which is a distance from a center in the longitudinal direction of the photosensitive member. Here, θ is an angle of incidence of the laser beam with respect to the photosensitive member. In FIG. 8B, there is shown a magnification at each image height with a magnification at an image height of 0 mm being 1. In order to express the characteristic of the changing magnification as shown in FIG. 8B, for example, a table of magnification information for each pixel may be prepared to address the problem. However, in order to prepare the table of the magnification information, a capacity of a memory for the number of pixels in one line in the main scanning direction is required, and there is a problem of an increased hardware scale.
Moreover, when the gradation is expressed in a digital PWM method, the gradation is quantized in units obtained by dividing a pixel, and hence a quantization error appears as a gradation error. For example, as illustrated in FIG. 8C, with respect to the pixel of part (a) divided by 8, due to the insertion-extraction positions of the divided pixels as indicated by the black circles in parts (b) and (c), the gradation is changed to an increased density in part (b) and to a reduced density in part (c). When the optical system is corrected using the table of the magnification information for each pixel, the same gradation error is arranged at the same position in the main scanning direction, and hence is visually conspicuous. Moreover, even with an fθ lens, which is configured to guide a laser beam deflected by a rotary polygon mirror 204 to a photosensitive drum 102, when an fθ lens having low accuracy is used, similar problems occur.